BOULDER, Colo.- The Geological Society of America's May issue of GEOLOGY contains a number of newsworthy items. Topics include formation of Genovesa Island on the Galapagos Archipelago and insights regarding the island's conspicuous absence of lava lizards; comparison of lunar and South African rock samples and their formation following impact events; new evidence of very early animal life on Earth; impacts of industrial SO2 and other contaminants on boreal-forest trees' consumption of CO2; analysis of 1500-year climate variability cycles in North America and their relationship to changes in ecosystems; and discussion of plate tectonics as a self-organized system and its relationship to mantle convection. The May GSA TODAY science article addresses the Holocene Black Sea-Mediterranean Sea reconnection, offering evidence that contradicts the Noah's Flood hypothesis.

Highlights are provided below. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY or GSA TODAY in stories published. Contact Ann Cairns for copies of articles and for additional information or other assistance.

GEOLOGY

Plate tectonics is one of the most powerful theories in earth sciences and it has explained many geological features by the interactions of tectonic plates. No theory, however, predicts the number, size or shapes of the plates and how they influence mantle convection. If the plate tectonic system is a self-organized system, receiving heat and matter from the mantle, then it will organize by different rules than if mantle convection provides the template. It appears that twelve may be the optimal number of major plates and that ~15% of the surface of Earth is composed of broad deformed zones to accommodate global plate motions. Volcanic chains reflect stress patterns in the outer shell, not narrow convective plumes from the deep mantle.

Streams of mountainous oceanic islands, such as New Zealand and Taiwan, are sources of large amounts of suspended organic matter to the oceans. The short length, steep gradient, and mass wasting enable these rivers to transport water and sediment quickly from the mountains to the ocean. Because the residence time of this sediment may be very short compared to larger continental rivers, global terrestrial geochemical and particulate organic-carbon inputs to the ocean, calculated only from larger rivers, may be underestimated. Human activities, especially deforestation and forest harvesting, have probably increased the natural flux of organic carbon. Little to no data exist for many of these islands and thus a more detailed assessment awaits further measurements. The few data available suggest that these islands may contribute as much as 35% of the particulate carbon entering the world's oceans.

Metamorphism on the Moon: A terrestrial analogue in the Vredefort dome, South Africa?
Roger L. Gibson, et al., University of the Witwatersrand, Private Bag 3, P.O. Wits 2050, Johannesburg, South Africa. Pages 475-478.

Among the rock samples brought back from the Moon by the Apollo and Luna missions is a set of fine-grained metamorphic rocks that formed at temperatures significantly in excess of 1000 °C. These lunar granulites have traditionally been interpreted as the product of heating of the brecciated (broken) rocks beneath the many impact craters on the Moon by younger igneous intrusions or impact melt bodies. In the Vredefort dome in South Africa, however, fine-grained granulites displaying many similarities to the lunar granulites have been identified. Evidence indicates that the Vredefort granulites formed as a result of extremely high temperatures associated with the formation of the 2.02 Ga, 250-300-km-wide Vredefort impact structure. Rather than an intrusive or impact melt heat source, the authors propose that the metamorphism reflects extreme shock heating, and subsequent moderately slow cooling, of rocks beneath the centers of giant impact craters. A similar mechanism may account for the lunar granulites.

Genovesa Island, renowned for both its abundant marine wildlife and mysterious origins, is a small volcano located on the northern periphery of the Galápagos Archipelago. Ocean-island volcanoes, such as the Hawaiian chain, are generally believed to originate as products of plumes of hot rock rising from deep in the mantle. Several observations based on our recent field study and geochemical analyses suggest that Genovesa Island may not have originated as a direct product of the Galápagos plume, as previously believed, but instead from the serendipitous proximity of the Galápagos mantle plume to a mid-ocean ridge; the Galápagos spreading center is <100 km north of Genovesa Island. The near-ridge origin of Genovesa Island and its surprisingly young age may shed some light on a longstanding question about Galápagos biogeography. Lava lizards (Microlophus) are found everywhere in the Galápagos Archipelago except for remote Wolf and Darwin Islands, a few isolated rocks, and Genovesa. The lizards' conspicuous absence from Genovesa has been attributed to the lack of prevailing ocean currents to carry it to the island. Microlophus, however, is observed on both nearby Pinta and Marchena Islands. If Genovesa is one of the more recent structures formed in the northern Galápagos, the failure of Microlophus to colonize the island may be related to the volcano's youth and its unique volcanic origins.

Is tectonic uplift or climate change the primary driving force behind continent-scale erosion? Over the past 13 m.y., erosion from tropical South America to the Atlantic Ocean reflects both tectonic events in the Andes and long-term changes in tropical climate. Based on the composition of material deposited at Ceara Rise off the coast of Brazil, the authors infer that tropical South America has undergone cycles of alternating wet and dry climate. Marine terrigenous accumulation rates are empirically related to these regional climate changes over the full length of the record, with arid climates favoring large-scale erosion. Tectonic events that occur ca. 8 and 4.5 Ma are also correlated to increased erosion, perhaps due to drainage-area expansion into more arid regions at these times. Changes in erosion rates from 13 to 8 Ma are not related to any known tectonic events or changes in high-latitude or global climate, thus, regional climate change appears to be the dominant control on erosion in tropical South America, with regional tectonics playing a secondary role.

Animal life evolved in the sea and then moved onto land. A unit of sandstone in southeastern Ontario, Canada, contains fossilized trackways that provide evidence for a very early step in this process. The sandstone was originally deposited as windblown sand dunes, probably in a seaside dune field. These rocks are ca. 470-500 Ma (Late Cambrian to Early Ordovician), nearly 40 m.y. older than the previously oldest evidence of animal life on land. The trackways were made by arthropods that possessed several pairs of legs. The way the animals walked indicates that they were still adapted for life under water. Therefore, they probably came onto land only briefly, perhaps to feed, reproduce, or escape predators.

It was previously proposed that boreal forests accommodate increasing amounts of atmospheric CO2. However, the effects of potentially toxic gases on photosynthesis had never been evaluated. In this article the authors' results suggest that trees affected by SO2, or other contaminants such as ozone, significantly lower their CO2 consumption. Given that large portions of forests are stressed by increasing amounts of atmospheric pollutants, the global response might equate to a much decreased rate of photosynthesis. Therefore, the capacity of the boreal forests to sequester CO2 produced by humans might not be as high as hoped for.

The authors analyzed how climate has varied since the glaciers retreated from North America. They studied pollen diagrams from across North America, which record changes of the vegetation in response to climate variations. The authors show that before the current warming caused by human activities, there were major reorganizations in the vegetation across all of North America occurring roughly every 1500 yr, synchronous with changes in the climate identified previously in ice cores and ocean sediments. These climate changes, probably caused by variations in solar output, caused large-scale impacts on North American ecosystems and perhaps societies. These results suggest that variations in the climate can occur abruptly and have rapid and significant impacts when the climate flips from one regime to another. The present human-induced CO2 global warming effect is superimposed on these natural climate incursions.

Controversy surrounds reconnection of the Black Sea and Mediterranean during Holocene sea-level rise. In their paper in Marine geology (1997; and numerous other publications), Bill Ryan, Walter Pitman, and coworkers have proposed that the reconnection took place via a catastrophic flood of Mediterranean water into the Black Sea at ~7.5 ka, and they suggest that this was the historical basis for the biblical story of Noah's Flood. In contrast, the Aksu et al. GSA Today article suggests a more complex and progressive reconnection over the last 12,000 years. Today, the Black Sea exports considerably more brackish water than the saline inflow it receives from the Mediterranean and there is a stratified, two-layer flow that has a strong affect on aquatic life and seabed sediments. The "Marmara Sea Gateway" (narrow straits of Dardanelles and Bosphorus, and deep intervening Marmara Sea) provides a set of natural flow valves (and sediment traps) that in principle should contain a record of the reconnection. Using ~7500 line-km of seismic profiles, 65 soft-sediment cores, and 43 radiocarbon dates, these authors recognize a 10,000-11,000 yr history of low surface-water salinities in the Marmara Sea and northern Aegean Sea. Several lines of evidence indicate persistent Black Sea outflow across the Bosphorus Strait rather than any catastrophic event. In particular, seismic data reveal a climbing delta on the middle shelf south of the Bosphorus exit that was active only from ~10-9 ka and that records flow out of the Black Sea. Ryan's and Pitman's argument for a catastrophic Black Sea flood hinges on the rapid first appearance of euryhaline (Mediterranean) mollusks on Black Sea shelves at ~7.5 ka. However, this paper proposes that the colonization was not a consequence of catastrophic flooding but rather the outcome of a slow establishment of two-way flow in the Bosphorus and a time lag during which the fresher waters of the deep Black Sea were replaced by more saline inflow, eventually allowing marine organisms to colonize the Black Sea shelves.